Strobel, Darrell F.

Becker, Tracy M.

Grava, Cesare

Abstract [en]

We report far-ultraviolet observations of Europa in transit of Jupiter obtained with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope on six occasions between 2014 December and 2015 March. Absorption of Jupiter's bright hydrogen Ly alpha dayglow is detected in a region several moon radii above the limb in all observations. The observed extended absorption provides the first detection of an atomic hydrogen corona around Europa. Molecular constituents in Europa's global sputtered atmosphere are shown to be optically thin to Lya. The observations are consistent with a radially escaping H corona with maximum densities at the surface in the range of (1.5-2.2) x 10(3) cm(-3), confirming the abundances predicted by Monte Carlo simulations. In addition, we search for anomalies around the limb of Europa from absorption by localized high H2O abundances from active plumes. No significant local absorption features are detected. We find that an H2O plume with line-of-sight column density in the order of 10(16) cm(-2), as inferred by Roth et al. would not be detectable based on the statistical fluctuations of the transit measurements, and hence is not excluded or further constrained. The presence of plumes with line-of-sight column densities of >2 x 10(17) cm(-2) can be excluded at a 3-sigma level during five of our six observations.

Retherford, Kurt D.

Becker, Tracy M.

Molyneux, Philippa

Saur, Joachim

Abstract [en]

Far-ultraviolet observations of Ganymede's atmospheric emissions were obtained with the Space Telescope Imaging Spectrograph (STIS) onboard of the Hubble Space Telescope (HST) on several occasions between 1998 and 2014. We analyze the Lyman-alpha emission from four HST campaigns in order to constrain the abundance and variation of atomic hydrogen in Ganymede's atmosphere. We apply a forward model that estimates surface reflection and resonant scattering in an escaping corona of the solar Lyman-alpha flux, taking into account the effects of the hydrogen in the interplanetary medium. The atmospheric emissions around Ganymede's disk derived for the observations taken between 1998 and 2011 are consistent with a hydrogen corona in the density range of (5-8) x 10(3) cm(-3) at the surface. The hydrogen density appears to be generally stable in that period. In 2014, Ganymede's corona brightness is approximately 3 times lower during two observations of Ganymede's trailing hemisphere and hardly detectable at all during two observations of the leading hemisphere. We also investigate extinction of Ganymede's coronal emissions in the Earth's upper atmosphere or geocorona. For small Doppler shifts, resonant scattering in the geocorona of the moon corona emissions can effectively reduce the brightness observed by HST. In the case of the 2014 leading hemisphere observations, an estimated extinction of 80% might explain the non-detection of Ganymede's hydrogen corona. Geocoronal extinction might also explain a previously detected hemispheric difference from Callisto's hydrogen corona.

Abstract [en]

Non-thermal echoes in incoherent scatter radar observations are occasionally seen in the high-latitude ionosphere. Such anomalous echoes are a manifestation of plasma instabilities on spatial scales matching the radar wavelength. Here we investigate the occurrence of a class of spatially localized anomalous echoes with an enhanced zero Doppler frequency feature and their relation to auroral particle precipitation. The ionization profile of the E region is used to parametrize the precipitation, with nmE and hmE being the E region peak electron density and the altitude of the peak, respectively. We find the occurrence rate of the echoes to generally increase with nmE and decrease with hmE, thereby indicating a correlation between the echoes and high-energy flux precipitation of particles with a high characteristic energy. The highest occurrence rate of > 20% is found for hmE = 109 km and nmE D 10(11.9) m(-3), averaged over the radar observation volume.

Whiter, Daniel K.

Abstract [en]

High-resolution multispectral optical and incoherent scatter radar data are used to study the variability of pulsating aurora. Two events have been analysed, and the data combined with electron transport and ion chemistry modelling provide estimates of the energy and energy flux during both the ON and OFF periods of the pulsations. Both the energy and energy flux are found to be reduced during each OFF period compared with the ON period, and the estimates indicate that it is the number flux of foremost higher-energy electrons that is reduced. The energies are found never to drop below a few kilo-electronvolts during the OFF periods for these events. The high-resolution optical data show the occurrence of dips in brightness below the diffuse background level immediately after the ON period has ended. Each dip lasts for about a second, with a reduction in brightness of up to 70% before the intensity increases to a steady background level again. A different kind of variation is also detected in the OFF period emissions during the second event, where a slower decrease in the background diffuse emission is seen with its brightness minimum just before the ON period, for a series of pulsations. Since the dips in the emission level during OFF are dependent on the switching between ON and OFF, this could indicate a common mechanism for the precipitation during the ON and OFF phases. A statistical analysis of brightness rise, fall, and ON times for the pulsations is also performed. It is found that the pulsations are often asymmetric, with either a slower increase of brightness or a slower fall.

Feldman, P. D.

Saur, J.

Spencer, J. R.

Strobel, D. F.

Abstract [en]

We report far ultraviolet observations of Ceres obtained with the Cosmic Origin Spectrograph (COS) of the Hubble Space Telescope in the search for atomic emissions from an exosphere. The derived brightnesses at the oxygen lines at 1304 Å and 1356 Å are consistent with zero signals within the 1σ propagated statistical uncertainties. The OI 1304 Å brightness of 0.12 ± 0.20 Rayleighs can be explained by solar resonant scattering from an atomic oxygen column density of (8.2 ± 13.4) × 1010 cm-2. Assuming that O is produced by photodissociation of H2O, we derive an upper limit for H2O abundance and compare it to previous observations. Our upper limit is well above the expected O brightness for a tenuous sublimated H2O exosphere, but it suggests that H2O production with a rate higher than 4 × 1026 molecules s-1 was not present at the time of the COS observation. Additionally, we derive an extremely low geometric albedo of ∼1% in the 1300 Å to 1400 Å range.

Abstract [en]

We analyze a large set of far ultraviolet oxygen aurora images of Europa's atmosphere taken by Hubble's Space Telescope Imaging Spectrograph (HST/STIS) in 1999 and on 19 occasions between 2012 and 2015. We find that both brightness and aurora morphology undergo systematic variations correlated to the periodically changing plasma environment. The time variable morphology seems to be strongly affected by Europa's interaction with the magnetospheric plasma. The brightest emissions are often found in the polar region where the ambient Jovian magnetic field line is normal to Europa's disk. Near the equator, where bright spots are found at Io, Europa's aurora is faint suggesting a general difference in how the plasma interaction shapes the aurora at Io and Europa. The dusk side is consistently brighter than the dawnside with only few exceptions, which cannot be readily explained by obvious plasma physical or known atmospheric effects. Brightness ratios of the near-surface OI] 1356 Å to OI 1304 Å emissions between 1.5 and 2.8 with a mean ratio of 2.0 are measured, confirming that Europa's bound atmosphere is dominated by O2. The 1356/1304 ratio decreases with increasing altitude in agreement with a more extended atomic O corona, but O2 prevails at least up to altitudes of ∼900 km. Differing 1356/1304 line ratios on the plasma upstream and downstream hemispheres are explained by a differing O mixing ratio in the near-surface O2 atmosphere of ∼5% (upstream) and ≲1% (downstream), respectively. During several eclipse observations, the aurora does not reveal any signs of systematic changes compared to the sunlit images suggesting no or only weak influence of sunlight on the aurora and an optically thin atmosphere.

Tsamsakizoglou, M.

Abstract [en]

A large portion of CubeSat projects have either been demonstrations or educational missions, where the science or operational concept has not been in focus. For efficient use of CubeSat platforms and realization of efficient services, either for scientific or commercial purposes, a full end-to-end design is needed, where the operational concept as well as a focused scientific or commercial rationale is taken into consideration. The SEAM project (funded within European Union’s Seventh Framework Programme) addresses parts of this challenge and develops operational concepts as well as on-board systems for scientific missions. The SEAM platform is using S-Band for downlink and uplink and is fully compliant to the CCSDS standards for satellite link services thus allowing compatibility with a global commercial ground station network. The project is led by the Royal Institute of Technology KTH and SSC is an industrial partner in the consortium. The 3U SEAM CubeSat is designed with an operational concept that includes on-board selection of data to download with earth in the loop, and flexible ground network scheduling. The spacecraft S-Band transceiver is full duplex with a downlink data-rate capability of 3 Mbps and uplink capability up to 100 Kbps. The communication link is CCSDS compatible in both directions, and operates with COTS multi-mission ground station equipment. A newly developed on-board module, that integrates mass memory and CCSDS functionality with a direct transceiver interface, is being demonstrated in the project. The data link layer of the CCSDS standards is implemented in hardware while the network layer and the data storage coordination in the mass memory are handled by software. This functionality partitioning ensures high data throughput and performance while providing flexibility in data collection and handling. It is noted that although the satellite is small, the complexity of such spacecraft is fully comparable to scientific microsatellites and its communication systems and operational concept use technology, equipment and procedures often found in much bigger satellites. The SEAM CubeSat is planned to be launched in early 2017 and the presentation will include the latest news from the mission operations.

Tsamsakizoglou, M.

Abstract [en]

A large portion of CubeSat projects have either been demonstrations or educational missions, where the science or operational concept has not been in focus. For efficient use of CubeSat platforms and realization of efficient services, either for scientific or commercial purposes, a full end-to-end design is needed, where the operational concept as well as a focused scientific or commercial rationale is taken into consideration. The SEAM project (funded within European Union’s Seventh Framework Programme) addresses parts of this challenge and develops operational concepts as well as on-board systems for scientific missions. The SEAM platform is using S-Band for downlink and uplink and is fully compliant to the CCSDS standards for satellite link services thus allowing compatibility with a global commercial ground station network. The project is led by the Royal Institute of Technology KTH and SSC is an industrial partner in the consortium. The 3U SEAM CubeSat is designed with an operational concept that includes on-board selection of data to download with earth in the loop, and flexible ground network scheduling. The spacecraft S-Band transceiver is full duplex with a downlink data-rate capability of 3 Mbps and uplink capability up to 100 Kbps. The communication link is CCSDS compatible in both directions, and operates with COTS multi-mission ground station equipment. A newly developed on-board module, that integrates mass memory and CCSDS functionality with a direct transceiver interface, is being demonstrated in the project. The data link layer of the CCSDS standards is implemented in hardware while the network layer and the data storage coordination in the mass memory are handled by software. This functionality partitioning ensures high data throughput and performance while providing flexibility in data collection and handling. It is noted that although the satellite is small, the complexity of such spacecraft is fully comparable to scientific microsatellites and its communication systems and operational concept use technology, equipment and procedures often found in much bigger satellites. The SEAM CubeSat is planned to be launched in early 2017 and the presentation will include the latest news from the mission operations.

Tuttle, S.

Grydeland, T.

Abstract [en]

Measurements of naturally enhanced ion acoustic line (NEIAL) echoes obtained with a five-antenna interferometric imaging radar system are presented. The observations were conducted with the European Incoherent SCATter (EIS-CAT) radar on Svalbard and the EISCAT Aperture Synthesis Imaging receivers (EASI) installed at the radar site. Four baselines of the interferometer are used in the analysis. Based on the coherence estimates derived from the measurements, we show that the enhanced backscattering region is of limited extent in the plane perpendicular to the geomagnetic field. Previously it has been argued that the enhanced backscatter region is limited in size; however, here the first unambiguous observations are presented. The size of the enhanced backscatter region is determined to be less than 900 x 500 m, and at times less than 160m in the direction of the longest antenna separation, assuming the scattering region to have a Gaussian scattering cross section in the plane perpendicular to the geomagnetic field. Using aperture synthesis imaging methods volumetric images of the NEIAL echo are obtained showing the enhanced backscattering region to be aligned with the geomagnetic field. Although optical auroral emissions are observed outside the radar look direction, our observations are consistent with the NEIAL echo occurring on field lines with particle precipitation.

Abstract [en]

High-resolution multimonochromatic measurements of auroral emissions have revealed the first optical evidence of coexisting small-scale auroral features resulting from separate high- and low-energy populations of precipitating electrons on the same field line. The features exhibit completely separate motion and morphology. From emission ratios and ion chemistry modeling, the average energy and energy flux of the precipitation is estimated. The high-energy precipitation is found to form large pulsating patches of 0.1Hz with a 3Hz modulation, and nonpulsating coexisting discrete auroral filaments. The low-energy precipitation is observed simultaneously on the same field line as discrete filaments with no pulsation. The simultaneous structures do not interact, and they drift with different speeds in different directions. We suggest that the high- and low-energy electron populations are accelerated by separate mechanisms, at different distances from Earth. The small-scale structures could be caused by local instabilities above the ionosphere.